Ceramic compounds such as the nitrides of boron, aluminum and silicon (BN, AlN, and Si.sub.3 N.sub.4) and the carbides of boron and silicon (B.sub.4 C and SiC) have high thermal stability and are impervious to chemical attack, properties which make them commercially important. However, their related properties of insolubility and infusibility make it difficult to process these materials into useful end products.
Ceramic compounds provide protective coatings for less stable structural materials, thereby expanding their useful applications. While the formation of composite materials utilizing alumina fibers is well known, alumina can form an eutectic at elevated temperatures when in an oxide matrix. Carbon fibers possess many desirable properties, including high temperature stability and mechanical strength. Unfortunately, unprotected carbon fibers are unstable in oxidizing atmospheres. However, articles made of carbon or alumina and coated with ceramic, such as BN or AlN, remain intact, thus retaining their mechanical strength on exposure to conditions at which the uncoated material would melt or oxidize. Present methods of forming a ceramic coating on these materials usually require chemical vapor deposition which is costly and difficult to control.
A readily processible precursor, which upon pyrolysis could be transformed into a ceramic, would offer specific potential for coating fibers, irregular shapes, producing thin films, and also as a binder for ceramic powders, eliminating the use of additives or sintering aids.
Soluble processible ceramic precursors of particular commercial interest are those which can be pyrolyzed to yield materials such as SiC, Si.sub.3 N.sub.4, as well as materials such as BN, AlN, B.sub.4 C and AlNBN. It is important, however, in order to produce a ceramic with desirable properties, that these precursors can be pyrolyzed to a pure ceramic, free of carbon and other impurities that affect its physical properties. The inventors have produced soluble and fusible borazine polymers capable of transformation into carbon-free boron nitride by the synthesis of B-triamino-N-tris (trialkylsilyl) borazines and silylamino-substituted borazines, and subsequent thermal condensation of these compounds into preceramic polymers. These soluble processible preceramic polymers, which produce pure BN ceramic on pyrolysis, are the subject of U.S. Pat. Nos. 4,581,468 and 4,707,556 to Paciorek et al.
Certain ceramic compounds frequently provide superior durability and heat resistance when combined with another in a defined stoichiometry. Soluble processible preceramic polymers comprising defined molar ratios of SiC and Si.sub.3 N.sub.4 and which produce mixed SiC and Si.sub.3 N.sub.4 on pyrolysis are the subject of U.S. Pat. No. 4,719,273 to Seyferth et al.
A readily processible precursor which upon pyrolysis can be transformed into a novel pure mixed ceramic offers a potential for applications which cannot be met by a single material such as either AlN or BN.
It is therefore a principal object of the invention to provide a suitable precursor of a mixed boron nitride and aluminum nitride ceramic which is soluble in organic solvents, which thus can be used to coat fibers, densify porous materials, or form ceramic coatings including thin films, on surfaces of materials.
It is also an object of the invention to provide processes for transforming the mixed boron nitride and aluminum nitride ceramic precursor into coatings and shaped articles of pure mixed boron and aluminum ceramic.
It is also an object of the present invention to provide a processible precursor for a novel mixed boron nitride and aluminum nitride ceramic having new and useful properties, whose subsequent pyrolysis provides a high yield of pure ceramic.